1. Field of the Invention
The present invention relates generally to amplifiers, and more specifically to a feedback biasing technique for a stage of an amplifier with a feedback control loop having low gain.
2. Related Art
Amplifiers are often employed to amplify (enhance the signal level/strength of) signals. In general, an amplifier amplifies an input signal to generate an amplified output signal. For example, amplifiers are employed in, but not limited to, radio receivers and transmitters, Analog to Digital Converters (ADCs), etc.
An amplifier may contain multiple stages, with each stage performing a corresponding desired operation, as is well known in the relevant arts. For example, an earlier stage (in the signal path) may be optimized to provide voltage/current amplification, while a subsequent stage (for example, a class AB stage) may be optimized to provide amplification (e.g., power amplification) while maintaining a high efficiency.
A stage of an amplifier generally contains active components that are biased to suitable operating points. In general, an active component is designed to add signal power to an input signal and provide a corresponding output upon appropriate biasing. Such biasing generally entails providing suitable voltage/current at corresponding terminals (e.g., gate/control terminal in case of components such as transistors etc.) to enable a desired amplification, as is well known in the relevant arts.
Feedback biasing is often used associated with such active components of a stage. Feedback biasing is a biasing technique in which a bias voltage/current (or a voltage/current proportional to the bias voltage/current) provided to one or more components is sensed, and adjusted to ideally maintain the bias voltage/current at a desired level.
The sensing and adjustment may be performed using feedback techniques by a feedback control loop, with the loop designed to have a corresponding gain. For example, transistors in a class AB stage of an amplifier may be biased by a feedback control loop with a desired gain to maintain the bias voltages/currents for the corresponding components of the stage, as is also well known in the relevant arts.
The gain of the feedback control loop (products of all the input-to-output gains of individual components in the loop) may depend on several factors, including the type of component(s) used in the loop. The presence of certain types of components in the loop may cause the loop gain to be low (lower than otherwise/ideally desired). Consequently, the bias voltages/currents may not be maintained at desired levels, often causing distortions in the output signal of the amplifier.
Several aspects of the present invention enable providing a feedback biasing technique for a stage of an amplifier that uses a feedback control loop having low gain, which addresses at least the distortion aspect noted above.